Electric propulsion for spacecraft is highly desirable for a variety of reasons, and, through the years, many designs for electric engines for spacecraft have been proposed and implemented. Two important categories of electric spacecraft engines are the ion engines and the plasma engines.
Before the advent of electric spacecraft engines, chemical rockets were the only technology available for spacecraft propulsion, and, the early decades of space exploration were based almost entirely on chemical rockets. An important difference between chemical rockets and electric engines is that, with chemical rockets, fuel and propellant are one and the same, but not so with electric propulsion. In this disclosure, the term “fuel” is used exclusively to refer to the source of energy for a propulsion system, while the term “propellant” is used exclusively to refer to the mass that is expelled by a propulsion system.
Most spacecraft propulsion systems accomplish their task by exploiting the principle of conservation of momentum. In accordance with that principle, if a mass is expelled by a spacecraft, there will be a force acting on the spacecraft while the mass is being expelled. Such force is referred to as “thrust” and is in proportion to the amount of mass that is expelled and in proportion to the velocity at which the mass is expelled. The higher the rate at which mass is expelled, the higher the thrust. Similarly, the faster the velocity of expulsion of the mass, the higher the thrust.
From the principle of conservation of momentum, as described in the previous paragraph, it might seem that a high velocity of expulsion is desirable because it yields higher thrust with less mass being expelled. However, the higher velocity comes at the cost of higher energy. In particular, calculations show that the power that must be expended to achieve a certain amount of thrust increases in proportion to the velocity of expulsion.
In spacecraft design, reducing the amount of propellant that needs to be carried is highly desirable. Such a reduction can be achieved by increasing the velocity of expulsion of the propellant. But reducing the amount of energy required by the spacecraft is also highly desirable, such that, in each space mission, a compromise must be struck between propellant mass and required energy. Such a compromise depends on the specific parameters of the mission and may be different in different parts of the mission. Therefore, it is advantageous to have a propulsion system wherein the velocity of expulsion of the propellant can be adjusted as needed to achieve such a compromise.
With chemical rockets, the velocity of expulsion of the propellant is limited by the amount of energy available from chemical reactions. Generally, the velocity of expulsion is much less than the optimum in most circumstances. That's why there is no benefit, with chemical rockets, in carrying propellant in addition to the chemical fuel needed for providing the chemical energy. Calculations show that best results are obtained by using the spent chemical fuel as propellant without mixing in additional propellant. That's also why the mass of a space rocket sitting on the launch pad is so much larger than what eventually makes it into orbit. Most of that mass is fuel.
Electric spacecraft engines are advantageous because fuel and propellant are separate. In particular, energy is supplied to the engine as electricity, and the fuel from which the electricity is generated can come from, for example, a nuclear reactor or a radioisotope source. Both such forms of fuel yield much more energy, per unit mass, than chemical fuels, such that there is no need to use the spent fuel as propellant. Better yet, for spacecraft that are close enough to the Sun, there is the option of generating electricity with solar panels. In such a case, the fuel is located in the Sun, and the spacecraft does not need to carry any fuel.
One potential benefit of electric engines is the opportunity to adjust the velocity of expulsion of the propellant. The feasibility of such an adjustment depends on the design of the electric engine, and some designs are better than others in that respect.
There are two major categories of electric engines known in the art: plasma engines and ion engines. With both categories, the propellant is prepared for expulsion by first ionizing it. The ionized propellant forms a plasma, which is a state of matter wherein atoms have lost one or more electrons, thereby becoming positive ions; and wherein the lost electrons remain mixed in with the ions, such that the overall mixture has no net electric charge.
Plasma engines heat the plasma by any of a variety of techniques well known in the art. As with any substance, heating a plasma means that plasma particles are accelerated, such that their kinetic energy (KE) increases. Heating implies that the resulting motion of plasma particles is random, so that different particles have different kinetic energies in accordance with a random distribution, and the direction of motion of the particles is also random with no preference for any particular direction of motion. Some of the heated plasma is then allowed to escape, forming the expelled propellant. Generally, the temperature of the plasma determines the average velocity with which the particles escape, which is the velocity of expulsion.
In contrast, with ion engines, ions and electrons in the plasma are separated via electric fields without heating the plasma. Electric fields are further used to accelerate the separated ions and electrons for expulsion. Ions and electrons are then expelled separately, and they recombine outside the spacecraft, forming the expelled propellant.
Plasma engines and ions engines have different advantages and disadvantages. With ion engines, the use of electric fields makes it possible to adjust the velocity of expulsion accurately by adjusting the electric fields that accelerate the ions and electrons. This capability maximizes the efficiency of utilization of propellant. Also, the size of the flow of electrons and ions can be similarly adjusted, such that the thrust generated by the engines can be adjusted easily and accurately. Another advantage is simplicity, because the conversion of electric energy into electric fields can be simply accomplished with metal electrodes that have specific shapes. However, there are disadvantages with ion engines. In particular, the accelerated ions might come in direct contact with the material of the electrodes and cause damage to the electrodes. And the negative electrons that leave the cathode and impinge on the anode also can cause damage to those electrodes. Generally, electrodes in ion engines have limited lifetime due to such wear.
Plasma engines are advantageous because they are not subject to the same wear mechanism as ion engines. But they are more complex because of the need to convert electric energy into plasma heating, and because the random motion of heated plasma particles, as they escape, means that not all particles have the optimal velocity of expulsion.
It would be desirable to have a type of electric spacecraft engine that combines the advantageous features of plasma engines and ion engines.